The present invention relates to turbine and compressor rotors having radial blade entry construction, and more particularly to rotors having circumferentially bifurcated blade roots.
In radial blade entry construction, the blades of an axial flow turbine or compressor rotor are assembled by radial positioning onto the rotor disc at an entry gap of a peripheral retainer member of the disc, then moved circumferentially around the disc to respective operating positions. In one form of such construction, the retaining feature is a peripheral groove having a T-shaped cross-section for engaging corresponding T-shaped roots of the blades. As described in U.S. Pat. No. 3,584,971 to the present inventor and incorporated herein by reference, it is desirable to include in the rotor structure one or more thin liner members interposed between blade roots in adjacent groups of the blades. In this form of the construction, the root liners are T-shaped, being retained in the groove analogously to the blades, except that the liners do not have counterparts of the platform portions of the roots that typically interlock with the disc for preventing lateral separation of opposite sides of the groove. The Tee-type liner can be installed from the notch or entry gap; however, by moving adjacent blades temporarily apart and because the platform portion of the root shape has been omitted, it is possible to install these liners at convenient locations around the periphery of the wheel by first orienting the liner in the plane of the disc so that it can be lowered into the groove. The liner is then rotated crosswise in the groove and the blades brought together against opposite sides of the liner.
In another and desirable form of the radial blade entry construction, the blade roots are bifurcated, the retaining feature being a flanged ring that is gripped on opposite sides by opposing legs of the blade roots. These circumferential bifurcated roots are variously called "outside," "straddle," "claw," or "dovetail" roots. A problem with this construction is that a liner or shim comforming to the bifurcated shape of the roots can only be installed at the notch or entry gap. This greatly complicates accurate measurement of the required shim thickness and requires the liners to be moved from the gap around the periphery of the disc to where they are needed, which is time-consuming and can cause liner distortion due to close clearances between the liners and the groove.
A recent development is that problems relating to corrosion in steam turbines have led to the evaluation and use of corrosion-resistant surface protection or coating systems. Corrosion attacks all surfaces of the blade, including the root. The root is a highly stressed part of the blade, being required to carry centrifugal direct, bending, shear and bearing stresses, as well as steam bending and associated vibratory stresses. The root is also a location where numerous stress concentrations occur. Corrosion pitting and corrodent buildup and expansion have been known to be so severe that the blades could only be removed for maintenance by machining them out. A corrosion-resistant coating system, using diffusion alloying or electroplating, for example, are an attractive means for avoiding corrosion attack; however, the coatings introduce dimensional variations such that it is extremely difficult or virtually impossible to assemble the blades without trimming off some of the coating and base metal of one or more of the roots unless root liners are used. Aside from the extra labor involved, the trimming of coated roots largely defeats the benefit of a surface protection system.
Thus there is a need for a bladed rotor structure incorporating bifurcated blade roots wherein groups of the blades are separated by a thin liner member interposed between adjacent blade roots, the structure providing installation of liners at convenient locations around the periphery of the wheel without requiring the liners to be inserted at a notch or entry gap of the rotor disc.